Comprehensive study of valorisation of exhausted olive pomace through the preparation of highly porous activated carbons

Abstract

Exhausted olive pomace (EOP), commonly known as alpeorujo, is a by-product of olive oil production. EOP is recognised for the environmental risk associated with its current management and/or valorisation methods. This work aims to recover this agro-industrial waste through its conversion into functional activated carbon (AC). To achieve a successful transformation, a simple three-stage process was studied: (i) mechanical pre-activation by preparing four different proportions of alpeorujo with KOH as an activating agent (1:1, 2:1, 3:1, and 4:1); (ii) thermochemical treatment at different pyrolysis temperatures (600–1000 °C); and (iii) chemical purification. The overall process yield varies between 2–15 % depending on the selected conditions. Innovatively, the liquid and gaseous effluents were characterised to propose an adequate valorisation approach. Additionally, mass, energy, and economic balances were carried out to analyse the profitability of the AC production process. ACs with a very large specific area (>1600 m2/g) and high carbon content (>90 %) were produced. The analysis of gaseous emissions from the thermochemical treatment resulted in a total yield of volatile organic compounds (VOCs) and volatile sulphur compounds (VSCs) of 874 and 2605 mg/kgAlpeorujo, respectively, which is a disadvantage from an environmental perspective. The elements removed during the purification process achieved a total of 217.97 mg/gAC, confirming the effectiveness of this stage. Finally, the cost of the alpeorujo-derived AC was estimated to be lower than €10/kg according to an economic analysis. As a novelty, this work demonstrates that highly porous AC of this type can be obtained without the usual impregnation and pre-carbonisation steps using ultra-low proportions of a dry activating agent.